1. Field of the Invention
The present invention relates to an exhaust emission control device and method for an internal combustion engine, and an engine control unit, for purifying exhaust emissions by temporarily trapping NOx contained in exhaust gases emitted from the engine and performing reduction of the trapped NOx.
2. Description of the Related Art
Conventionally, there has been disclosed an exhaust emission control device for an internal combustion engine, e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2006-207487. This internal combustion engine is a gasoline engine having an exhaust pipe, and a three-way catalyst and a NOx catalyst are arranged in the exhaust pipe in this order from the upstream side to the downstream side, for purifying HC and CO, i.e. reducing HC and CO emissions mainly during low temperature, and for purifying NOx, i.e. reducing NOx emissions, respectively. According to this exhaust emission control device, exhaust gases emitted from the engine during lean burn operation pass through the three-way catalyst and flow into the NOx catalyst to be temporarily trapped therein. Then, when the amount of trapped NOx reaches a predetermined value, rich spike is carried out in which the air-fuel ratio of a mixture supplied to the engine is controlled to a richer side than a stoichiometric air-fuel ratio. The rich spike causes unburned fuel components to be supplied to the NOx catalyst as reducing agent, whereby the NOx trapped in the NOx catalyst is reduced to be purified, i.e. changed into harmless ingredients. Further, the amount of reducing agent supplied to the NOx catalyst is cumulatively calculated from the start of rich spike, and when the cumulative value exceeds a threshold value set according to the amount of trapped NOx, it is judged that the reduction of NOx is complete, and the rich spike is terminated.
As described above, in the conventional exhaust emission control device, the cumulative calculation of the amount of reducing agent supplied to the NOx catalyst is started immediately after the start of rich spike. However, at the initial stage of the rich spike, since the lean burn operation has been carried out up to just before the start of rich spike, and hence a relatively large amount of oxygen remains in the exhaust gases, and a relatively large amount of oxygen has been stored in the three-way catalyst. Therefore, when such exhaust gases pass through the three-way catalyst, the reducing agent is oxidized by the oxygen and is consumed. Accordingly, when the cumulative calculation of the amount of reducing agent supplied to the NOx catalyst is started immediately after the start of rich spike, the calculated cumulative value becomes larger than an actual value. As a result, in a state where the reduction of NOx has not been completed and hence NOx remains in the NOx catalyst, the cumulative value exceeds the threshold valve to terminate the rich spike, which causes insufficient reduction of NOx, resulting in increased exhaust emissions.